The physico-chemical factors affecting the viscoelastic properties of tracheal secretions will be investigated using mucus obtained via the canine tracheal pouch model. Work will continue on fractionation of the macromolecular constituents of the mucus and correlation with its physical-chemical behavior. Exposure of animals to sulfur dioxide and to various pharmacological agents will also be assessed in terms of changes in viscoelastic properties of the mucus.